As control over the waveform of an inverter, 120-degree conduction waveforms is generally adopted from the viewpoint of simplicity in control. In a driving system for a brushless DC motor, switches of respective phases of the inverter are electrically conducted within 120 degrees in electrical angles although the electrical angle is spanned as wide as 180 degrees both on positive side and negative side respectively. No control is thus done in the remaining period of 60 degrees in electrical angles. During this non-controlled period, the inverter fails to output a desired voltage, so that the inverter uses the DC voltage at a low utilization rate. This low utilization rate causes a low voltage between the respective terminals of the brushless DC motor as well as narrows the working range of the inverter. The maximum rotational speed of the DC motor is thus obliged to be low.
On the other hand, a wide-angle control method, which widens an electrically conduction angle to over 120 degrees in electrical angles, is proposed because this method allows enlarging the working range of the inverter for increasing an output of the inverter controller (e.g. refer to Patent citation 1, for instance). Patent citation 1 discloses that a conduction range of a voltage-type inverter is set at a given range over 120 degrees and not greater than 180 degrees in electrical angles, so that a non-controlled period becomes as small as less than 60 degrees in electrical angles. As a result, the voltages between the respective terminals of the motor become greater, which widens the working range of the inverter.
In recent years, permanent magnets are embedded in a rotor to generate torque caused by reluctance in addition to torque caused by magnets for obtaining higher efficiency. This brushless DC motor allows increasing the torque without a need for increasing a motor current.
To use this reluctance torque more efficiently, a voltage phase of the inverter is advanced with respect to an induction-voltage phase of the motor. This is called a phase-advancing control method, which can also efficiently use a weak magnetic flux, thereby increasing output-torque.
A compressor employs an inverter controller which uses no sensors such as a Hall element, from the viewpoints of service condition, reliability and maintenance. The inverter controller employs a sensor-less method in which a pole position of a rotor is sensed from an induction voltage generated in stator windings. This sensor-less method uses the span of 60 degrees in electrical angles, i.e. the non-controlled period, and monitors induction voltages available at the respective terminals of the motor during the switch-off of the upper and lower arms, thereby sensing the pole position of the rotor.
A conventional inverter controller is described hereinafter with reference to the accompanying drawings. FIG. 7 shows a structure of the conventional inverter, and FIG. 8 shows characteristics of torque vs. rotational speed of the conventional inverter. Specifically, it shows the characteristics of wide-angle control. FIG. 8 tells that the maximum rotational speed increases at a wider conduction angle provided when the torque is kept at a constant level.
FIG. 9 shows timing charts of the signal waveforms of respective sections of the conventional inverter controller. The timing charts also indicate the processes of the respective sections and the characteristics at conduction angle of 150 degrees in electrical angles. In FIG. 7, three pairs of switching transistors connected in series, i.e. Tru and Trx, Try and Try, Trw and Trz are coupled between the terminals of DC power supply 001, thereby forming inverter circuit 002. Brushless DC motor 003 is formed of stator 003A and rotor 003B. Stator 003A is formed of four poles and distributed windings. Rotor 003B is an interior magnet type where permanent magnets 003N and 003S are embedded.
The connection points of respective pairs of the switching transistors are coupled to brushless DC motor 003 at respective terminals of stator windings 003U, 003V, and 003W of respective phases, forming a “wye” connection. The connection points of respective pairs of the switching transistors are also coupled to respective resistors 004U, 004V, and 004W forming a “wye” connection. Reflux diodes Du, Dx, Dv, Dy, Dw, and Dz are coupled between the collector and the emitter of respective switching transistors Tru, Trx, Trv, Try, Trw, and Trz, for a protection purpose.
Pole-position sensing circuit 010 is formed of differential amplifier 011, integrator 012, and zero-crossing comparator 013. A voltage at neutral point 003D of stator windings 003U, 003V, and 003W coupled together in the wye connection is supplied to an inverting input terminal of amplifier 011B via resistor 011A. A voltage at neutral point 004D of resistors 004U, 004V, and 004W coupled together in the wye connection is supplied directly to non-inverting input terminal of amplifier 011B. Resistor 011C is coupled between an output terminal and the non-inverting input terminal of amplifier 011B. Differential amplifier 011 is thus formed.
An output signal from the output terminal of differential amplifier 011 is supplied to integrator 012 formed of resistor 012A and capacitor 012B coupled together in series. An output signal from integrator 012 (i.e. a voltage at a connection point between resistor 012A and capacitor 012B) is supplied to a non-inverting input terminal of zero-crossing comparator 013.
A voltage at neutral point 003D is supplied to an inverting input terminal of zero-crossing comparator 013. An output terminal of zero-crossing comparator 013 outputs a pole-position sensing signal.
Differential amplifier 011, integrator 012 and zero-crossing comparator 013 form pole-position sensing circuit 010 which senses a pole position of rotor 003B of brushless DC motor 003. Pole-position sensing circuit 010 outputs the pole-position sensing signal to microprocessor 020. Microprocessor 020 then corrects the phases of the supplied pole-position sensing signal in order to measure a cycle, and set a phase advance angle as well as a conduction angle. Microprocessor 020 calculates a timer counting value per cycle of an electric angle for determining a commutation signal of respective switching transistors Tru, Trx, Trv, Try, Trw, and Trz.
Microprocessor 020 outputs a voltage instruction based on a rotational speed instruction, and performs pulse width modulation (PWM) to the voltage instruction. Microprocessor 020 controls a duty ratio, i.e. a ratio of ON vs. OFF of a PWM signal, based on a difference between the rotational speed instruction and an actual rotational speed, and outputs PWM signals for the three phases. Microprocessor 020 increases the duty ratio when the actual rotational speed is smaller than the rotational speed instruction, and reduces the duty ratio when the actual rotational speed is greater than the rotational speed instruction.
The PWM signal is supplied to driving circuit 030. Driving circuit 030 outputs driving signals to respective base terminals of switching transistors Tru, Trx, Trv, Try, Trw, and Trz.
A conduction work of the inverter controller discussed above is described hereinafter. In FIG. 9, induction voltages Eu, Ev, and Ew of phases U, V and W of brushless DC motor 003 vary while the respective phases shift by 120 degrees from each other. A differential amplifier output signal indicates a signal output from differential amplifier 011. A signal supplied from integrator 012 forms an integral waveform shaped by integrator 012. A supply of the integral waveform to zero-crossing comparator 013 prompts an output signal from zero-crossing comparator 013 to rise and then fall at the zero-crossing point of the integral waveform. This excitation switching signal is output as the pole-position sensing signal.
The rise and fall of the excitation switching signal prompt phase correction timer G1 to start, and the start of timer G1 prompts second phase correction timer G2 to start. Both of timers G1 and G2 advance inverter mode N, i.e. a commutation pattern, by one step.
A conduction timing of phase U can be calculated based on the induction voltage waveform of phase W, and an amount of phase advance of the inverter can be controlled by phase-correction timer G1. In FIG. 9, a phase advance angle is set at 60 degrees when conduction angle is 150 degrees in electric angles. Phase correction timer G1 thus counts a value corresponding to 45 degrees, and second phase correction timer G2 counts a value corresponding to 30 degrees in electric angles. As a result, the ON-OFF states of switching transistors Tru, Trx, Trv, Try, Trw, and Trz are controlled as shown in FIG. 9 in response to the respective inverter modes.
As discussed above, brushless DC motor 003 can be driven in a state where a conduction period is set between 120 degrees and 180 degrees, and a phase of inverter voltage can be advanced with respect to that of the induction voltage of the motor.
The rotation of rotor 003B generates an induction voltage at stator windings 003U, 003V, 003W, and the induction voltage can be sensed through the foregoing conventional structure. This induction voltage is shifted its phase by integrator 012 having a delay of 90 degrees, thereby sensing a position sensing signal corresponding to a magnetic pole of rotor 003B. Based on this position sensing signal, conduction timings to stator windings 003U, 003V, 003W are determined. Use of such integrator 012 as having a phase-delay of 90 degrees lowers the responsiveness to an abrupt acceleration or deceleration.
A position sensing circuit improved in responsiveness has been proposed (e.g. refer to Patent citation 2, for instance). Another conventional inverter controller disclosed in Patent citation 2 is described hereinafter with reference to the accompanying drawings. FIG. 10 shows a structure of another conventional inverter controller, and FIG. 11 shows timing charts of the signal waveforms of respective sections of the conventional inverter controller. The timing charts also indicate the processes of the respective sections.
In FIG. 10, resistors 101 and 102 are coupled in series between bus 103 and bus 104, and their common connection point, i.e. sensing terminal ON, supplies voltage VN of a virtual neutral point. Voltage VN is a half of the voltage of DC power supply 001, and the voltage of DC power supply 001 corresponds to a voltage of the neutral point of stator windings 105U, 105V, and 105W.
Respective non-inverting input terminals (+) of comparator 106A, 106B, and 106C are coupled to output terminals OU, OV, and OW via resistors 107, 108, and 109, respectively. Respective inverting input terminals (−) of the comparators are coupled to sensing terminal ON.
Respective output terminals of comparators 106A, 106B, and 106C are coupled to microprocessor 110, having a logic circuit therein, at its input terminals I1, I2, and I3. Outputs from output terminals 01 through 06 of microprocessor 110 drive switching transistors Tru, Trx, Trv, Try, Trw, and Trz via driving circuit 120.
Brushless DC motor 105 includes four poles and distributed windings. Rotor 105A forms a structure of surface mounted magnet, i.e. permanent magnets 105N and 105S are mounted to the surface of rotor 105A. Motor 105 is thus set in a state where conduction angle is set at 120 degrees and phase advance angle is set at 0 degree in electric angles.
The structure is further described with reference to FIG. 11. Terminal voltage Vu, terminal voltage Vv, and terminal voltage Vw indicate respectively the voltages across stator windings 105U, 105V, and 105W during a regular operation of motor 105. Assume that inverter circuit 140 supplies voltages Vua, Vva, and Vwa, and stator windings 105U, 105V, and 105W generate induction voltages Vub, Vvb, and Vwb. Assume that a conduction, occurring at an event of commutation switch, of any one of reflux diodes Du, Dx, Dv, Dy, Dw, or Dz of inverter circuit 140 will generate pulse-like spike voltages Vuc, Vvc, and Vwc. Then respective terminal voltages Vu, Vv, Vw form a waveform combined by supplied voltages Vua, Vva, Vwa, induction voltages Vub, Vvb, Vwb, and spike voltages Vuc, Vvc, Vwc respectively.
Output signals PSu, PSv, PSw from the comparators indicate the results of comparison done by comparators 106A, 106B, 106C between terminal voltages Vu, Vv, Vw and voltage VN at the virtual neutral point. In this case, output signals PSu, PSv, PSw are formed of signals PSua, PSva, PSwa which indicate a positive-negative and a phase of each one of induction voltages Vub, Vvb, Vwb, and output signals PSub, PSvb, PSwb corresponding to spike voltages Vuc, Vvc, Vwc.
Spike voltages Vuc, Vvc, Vwc are neglected by a wait-timer, so that output signals PSu, PSv, PSw indicate a positive-negative and a phase of each one of induction voltages Vub, Vvb, Vwb, as a result.
Microprocessor 110 recognizes six modes, A, B, C, D, E, and F as shown in the mode column based on the status of signals PSu, PSv, PSw output from the comparators, and then it outputs driving signals DSu through DSz with a delay of 30 degrees in electric angles from the instant of variation in levels of output signals PSu, PSv, PSw. Respective time T of each mode A through F indicates 60 degrees, and a half of the time of each mode A through F, i.e. T/2, indicates a delay time corresponding to 30 degrees in electric angles.
Microprocessor 110 thus senses the rotational position of rotor 105A of motor 105 based on the induction voltages generated at stator windings 105U, 105V, 105W in response to the rotation of rotor 105A. It also determines driving signals for the conduction to stator windings 105U, 105V, 105W, depending on the conduction mode and the timing, by detecting the variable time T of the induction voltages, and then supplies electricity to stator windings 105U, 105V, and 105W.
The foregoing structure thus differs from the conventional inverter controller disclosed in Patent citation 1, and since it needs no filter circuit, it can detect an induction voltage with higher sensitivity. As a result, starting characteristic can be improved, and the motor can be driven at a lower rotational speed. On top of that, since no filter circuit having a delay of 90 degrees is used, the motor can be controlled with a delay of as small as 30 degrees by combining first timer 122 and second timer 123. The responsiveness to an abrupt acceleration or deceleration can be thus improved.
Next, out-of-synchronous characteristics of voltages and conduction angles of an inverter controller is described hereinafter with reference to FIG. 12. FIG. 12 shows the out-of-synchronous characteristics of the voltages and the conduction angles of the inverter controller shown in FIG. 10. As FIG. 12 shows, in the case of a sharp fall of the voltage, a resistance to the out-of-synchronous decreases at a greater conduction angle. Characteristics similar to this one can be also observed when the voltage sharply rises.
Patent citation 1 proposes magnetic-pole-position sensing circuit 010 operable within a period of 180 degrees in electric angles. However, since circuit 010 employs a filter, a delay of 90 degrees in electric angles occurs, which causes a lower responsiveness to a variation in rotational speed such as an abrupt change in load. As a result, the motor sometimes falls in out-of-synchronous and halts its work.
Patent citation 2 proposes a position sensing circuit free from a delay of 90 degrees in electric angles. However, even this structure sometimes cannot sense the pole-position during a rotational variation, such as an abrupt change in load, and thus the motor may fall in out-of-synchronous. Such a phenomenon occurs in the following cases: (1) when wide-angle control which widens a conduction angle to over 120 degrees is carried out, (2) when phase-advance-angle control is carried out, this control method advances a voltage phase of the inverter with respect to a phase of the induction voltage of the motor, (3) when a width of the spike voltage is widened in order to obtain a higher efficiency by increasing an inductance through a greater number of turns of stator windings 105U, 105V, 150W. These cases incur a shorter position sensible period.
In the case of employing a concentrated winding in a stator of the motor in order to obtain a higher efficiency and to increase greater torque, when six poles are used instead of four poles, the position sensible period decreases to as small as ⅔ mechanical angles comparing with the case of using four poles. Therefore, the foregoing wide-angle control, the phase-advance-angle control, the increase in the number of turns, or the increase in the number of poles for incurring the shorter mechanical position sensible range, shortens the pole-position sensible period. Thus an occurrence of a variation in load, an instantaneous power interruption, or a variation in voltage will accompany an abrupt variation in rotation, so that the inverter controller fails to sense the pole position and the motor falls in the out-of-synchronous.
[Patent Citation 1]
    International Publication Pamphlet No. 95/27328[Patent Citation 2]    Japanese Patent Unexamined Publication No. H01-8890